Apple Watch Sapphire Crystal Glass. Apple Watch series 5 comes with sapphire crystal glass both on display and back. Requires an iPhone 5 or later.
Aluminum models have Ion-X glass displays. Check out my accessories vid. The Apple Watchs ion-X strengthened glass is more flexible.
The same material is found in Rolex and Breitling watches.
LOBINNI Mens Quartz Watch Leather Steel Waterproof Sapphire Glass Luxury Watch. Both models have an identical OLED display. According to test results ion-X strengthened glass is 25 times tougher than the sapphire crystal. Sapphire is scratch-resistant and tough.